Frequently a finished image may be set off to better effect by surrounding it with a border.
Borders are easy to make and can be produced in a variety of ways.
Here are some ideas to try using Adobe Photoshop.

Let's get started
Firstly, make certain that your image has a Background layer - if your image doesn't have one, go to Layers > New > Background from Layer / Layer from Background (CS) and one will appear.

The important thing to remember is that every time you expand the canvas size the space outside the image will be automatically filled by the background colour - or the colour selected in the Canvas Size dialogue box (CS).

As an example we'll begin by working with a horizontal (landscape format) A4 size image to which we'll add a white pencil-line around the picture, surrounded by a blue border.

Open your image
Go to:
Image > Image Size.
Make certain that the Resample Image box is not ticked.
Make certain that 'Constrain Proprtion' box is ticked
Set the Width to 26cm. (The Height will take care of itself).

Ensure that the Foreground and Background colours to the base of the toolbox are set to Foreground, blue (or the colour of your choice) and Background, white.

Expand the canvas area:
Go to:
Image > Canvas Size
Expand Width & Height by 4 pixels.
Use the drop down arrow alongside the Width and Height boxes to enable you to choose the pixel dimensions (rather than inches or cm's etc)The more pixels you choose, the wider the border will be.
Keep the image in the centre of the nine small squares in the Canvas Size dialogue box. This will ensure a regular border width around the outside.
Click OK

The image now has a narrow white pencil line surrounding it.
Press Control + 0 (zero) to show the whole image area.

Exchange the Background and Foreground colours by clicking on the double-ended curving arrow to the top r.h. of the Foreground/Background colours icon (near base of toolbox) or simply use the 'X' key on the keyboard - this changes or 'exchanges' (X-changes) the f/g & b/g colours and is a useful shortcut.

Return to Canvas Size, this time using the 'cm' dimensions; increase the Width and the Height by 2cms each. This is, in effect, 1cm for each side of the image.
Click OK and again view by Control + 0 (zero) and there you have it! - A bright new (blue) border surrounding your picture. See below.

Photoshop CS users please note that the CS Canvas Size dialogue box in enables more choices - you can choose whether to use Foreground, Background colours or a colour set by choice (Canvas Extension Colour > Other).